Add basic exception handling support (#2282)

This adds basic support for exception handling instructions, according
to the spec:
https://github.com/WebAssembly/exception-handling/blob/master/proposals/Exceptions.md

This PR includes support for:
- Binary reading/writing
- Wast reading/writing
- Stack IR
- Validation
- binaryen.js + C API
- Few IR routines: branch-utils, type-updating, etc
- Few passes: just enough to make `wasm-opt -O` pass
- Tests

This PR does not include support for many optimization passes, fuzzer,
or interpreter. They will be follow-up PRs.

Try-catch construct is modeled in Binaryen IR in a similar manner to
that of if-else: each of try body and catch body will contain a block,
which can be omitted if there is only a single instruction. This block
will not be emitted in wast or binary, as in if-else. As in if-else,
`class Try` contains two expressions each for try body and catch body,
and `catch` is not modeled as an instruction. `exnref` value pushed by
`catch` is get by `pop` instruction.

`br_on_exn` is special: it returns different types of values when taken
and not taken. We make `exnref`, the type `br_on_exn` pushes if not
taken, as `br_on_exn`'s type.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/wasm-interpreter.h b/src/wasm-interpreter.h
index f683f9b38..53345f0ab 100644
--- a/src/wasm-interpreter.h
+++ b/src/wasm-interpreter.h
@@ -1048,6 +1048,10 @@ public:
   Flow visitAtomicNotify(AtomicNotify*) { WASM_UNREACHABLE(); }
   Flow visitPush(Push*) { WASM_UNREACHABLE(); }
   Flow visitPop(Pop*) { WASM_UNREACHABLE(); }
+  Flow visitTry(Try*) { WASM_UNREACHABLE(); }
+  Flow visitThrow(Throw*) { WASM_UNREACHABLE(); }
+  Flow visitRethrow(Rethrow*) { WASM_UNREACHABLE(); }
+  Flow visitBrOnExn(BrOnExn*) { WASM_UNREACHABLE(); }
 
   virtual void trap(const char* why) { WASM_UNREACHABLE(); }
 };